The S&P 500 has been inching closer and closer toward its all-time closing high of 1565 reached in October 2007.

But Brian Amidei, a Palm Desert, Calif.-based managing director at HighTower Advisors, cautioned that all these new highs in the markets need to be taken with "a grain of salt" as the landscape today is very different than in 2007. "We have never seen this much Fed intervention and this low of an interest rate environment in any past markets."

Amidei said when the S&P 500 was at its peak in 2007, trailing 12-month corporate earnings were $89.35 a share, producing a price-earnings ratio of 17.5. Earnings today, meanwhile, are at $98.30 a share, with the market currently trading at an earnings multiple of about 14.8. If the S&P 500 were trading at the same earnings multiple as at its 2007 peak, the index would be at 1722, roughly 18% higher than the current level, he explained.

"You cannot argue with the price/earnings ratio as being cheap right now, but since we are in uncharted waters, I am recommending clients proceed with caution and add to the market on pullbacks," said Amidei.

Financial stocks rose ahead of the Federal Reserve's annual stress test results set to arrive after the closing bell Thursday.

Boeing shares were up 2.4% and jumped to their highest level since June 2008 after Reuters reported that U.S. safety regulators could approve in just days a plan to allow the aerospace giant to begin flight tests of the 787 Dreamliner with a fix for its volatile batteries.

Home Depot (HD) , Merck(MRK) and Exxon(XOM) shares were declining the most.

The S&P 500 was up more than 1 point, or 0.07%, at 1,542.58. The Nasdaq was higher by more than 2 points, or 0.09%, at 3,225.29.